In a conventional integrated circuit, as illustrated in FIG. 1, an output circuit emits signals IN1, IN2 from the internal circuit via an output line 1 to an output pad P.sub.1, the line 1 being typically made of a metal such as an aluminum or the like of low resistance in order to increase the emitting speed more or less. If a high surge caused by an ESD (Electrostatic Discharge) or the like is applied to this output circuit from the exterior, then the surge is applied to an N-channel transistor Q.sub.1 or Q.sub.2 from the output pad P.sub.1 through the line 1 without being most alleviated, to cause a punch-through to be emitted to the power supply (Vcc) or ground (GND).
Transistors Q.sub.1, Q.sub.2 are designed greater than the ordinary transistors within the internal circuit in order to alleviate the concentration of the electric field during the punch-through as well as to increase the output speed during the normal operation.
In this conventional output circuit, in view of the operating characteristics, the size of the transistor Q.sub.2 is often made smaller. In such a case, when the high surge is applied, the concentration of the electric field often causes the transistor Q.sub.2 to be broken down. In order to prevent this, conventionally, independently of the transistor Q.sub.2, a punch-through transistor Q.sub.3 whose source and gate are connected to the GND and whose drain is connected to the output line 1 respectively is provided to protect the transistor Q.sub.2, as shown in FIG. 1.
However, in this arrangement, if the high surge is instantaneously applied between the output pad P.sub.1, and the GND, since a voltage is directly applied between the drain and the gate of the transistor Q.sub.3, the gate insulating film of the transistor Q.sub.3 can be destroyed.
For an input circuit, since the surge applied to the input pad is alleviated to some degree by a protection resistor while it reaches the punch-through transistor, it does not suffer such an intensive electric field as the gate insulating film of the punch-through transistor of the output circuit, but there also exists a danger of being destroyed.